DMD's have found numerous applications in the areas of optical information processing, projection displays, and electrostatic printing. See references cited in L. Hornbeck, 128 X 128 Deformable Mirror Device, 30 IEEE Tran. Elec. Dev. 539 (1983).
A great number of the applications described in Hornbeck, supra, use DMD's operated in a bistable mode as described in U.S. Pat. No. 5,096,279, incorporated by reference herein. The details of '279 will be summarized in some detail herein, but briefly in the bistable mode of a DMD a deflectable beam or mirror may be deflected to one of two landing angles, .+-..theta..sub.L, by underlying electrodes to which an address voltage is applied. At either landing angle (.+-..theta..sub.L) an extremity of the deflectable mirror lies in contact with an underlying device substrate.
With further reference to '279, in order to lower the address voltage requirement, a bias voltage is applied to the mirror relative to the address electrodes. The bias voltage serves to create energy potential minima. The amount of bias determines whether the deflectable mirror and its associated address and bias circuitry operated in a monostable, tristable, or bistable mode corresponding respectively to one, three, or two energy potential minima. The required address voltage also varies with the amount of bias, and typically the bias voltage is chosen such that the address voltages may operate with 5 V CMOS limits. For example, a typical bistable DMD operated with no bias requires a 16 volt address. At a bias of -10 V the DMD is operating in the tristable mode and requires a +10 V address. At a bias of -16 V the DMD is operating in the bistable mode and requires only a +5 V address. It is clear in this example, that to be compatible with standard 5 V CMOS address circuitry, it is necessary to operate in the bistable mode, which requires bidirectional operation and addressing. When the bias voltage is applied to the deflectable mirror, further changes in the address electrodes within normal operating limits cause no change in state of the deflectable mirror because the address voltage is not sufficient to overcome the potential energy barrier between the stable state in which the mirror resides and the other stable state which exists in a bistable mode. In order to change stable states it is necessary to remove the bias voltage to allow the deflectable mirror to respond to the voltage of the address electrode.
It has been discovered with prior art DMD's that when a deflectable mirror is deflected and in contact with the landing pads on the DMD substrate, it is necessary to apply a high voltage, high frequency resonant reset sequence to allow the mirror's addressed state to change. The reset sequence was adopted to overcome sticking difficulties caused by Van der Waal's forces or surface contamination. These sticking difficulties cause the beam to resist changing states regardless of the condition of the address electrodes underneath the beam.